Plans alone are not sufficient to identify and locate services before starting work. They provide basic information on which to base a thorough site survey before work begins.
Plans can give an indication of the location, configuration and number of underground services at a particular site and should help subsequent tracing by detecting devices or locators. However, they are not always drawn accurately to scale and, even if they claim to be, you should not rely on them to obtain distances or depths. Accuracy may be further limited because:
- The position of reference points (e.g. the kerb line) may have changed since the plans were drawn.
- Re-grading of the surface may mean that the depths shown are now incorrect.
- Services, particularly cables, may have been moved without the knowledge of their owners/operators.
- In many cases service connections are not marked.
- Services marked as straight lines may, in practice, snake. Excessively long cables may have been laid in horizontal loops outside substations, switch rooms etc.
- Plans may show spare ducts.
- The routes of older services in particular may not have been recorded, so the absence of records should never be taken as proof that the area in question is free of underground services.
Those using or interpreting plans need to have sufficient knowledge and experience to understand what the plans show in order to determine the likely presence of services on the work site.